Variable cam timing systems operate to vary the timing between the camshaft and the crankshaft to optimize engine performance over the entire range of engine operation. Systems such as that described in U.S. Pat. No. 5,117,784 to Schechter et al., vary the timing between the camshaft and crankshaft to achieve improved idle stability, expanded torque curve and the RPM (revolutions per minute) range of the engine, full control of emission gases and elimination of certain emissions, and elimination of external exhaust gas recirculation components and circuitry.
It is known that optimal cam timing for fuel economy and emissions may be achieved by determining the timing as a function of engine speed and aircharge entering the engine in lbs/cylinder filling. Optimal cam timing for performance may be achieved by determining the cam timing as a function of engine speed and throttle position. Either of the aforesaid control methods can generate cam timing to achieve satisfactory fuel economy, emissions and performance for a particular altitude, usually sea level. However, as the altitude at which a vehicle is operated increases a control method calibrated for sea level operation provides less than optimal results because the aircharge entering the engine at a given throttle position decreases. Exclusive use of throttle position to determine cam timing causes too much retard and charge dilution at low aircharge levels. Exclusive use of aircharge to determine cam timing causes too much retard at high throttle angles and peak power is not achieved.
Accordingly, there is a need for a variable cam timing system which provides optimal fuel economy, emissions and performance at a variety of altitudes.